Cystine Auxotrophy in Human and Rodent leukemia cells is well-recognized phenomenon which often distinguishes such cells from their non-neoplastic counterparts. Our present evidence indicates that such cys- cells are unable to synthesize effectively, gamma-cystathionase a 43,000 dalton polypeptide which catalyzes the last step in the de novo cystine biosynthetic pathway. The molecular biological basis for this phenotypic abnormality is under investigation.